Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.
Some types of transmissions operate in a discrete number of different operating modes, each associated with a predetermined speed ratio between an input and an output. Such transmissions must periodically shift between the operating modes in order to adjust the speed ratio. Other types of transmissions, called Continuously Variable Transmissions (CVTs) are capable of establishing an infinite number of speed ratios within a predetermined range of speed ratios. CVTs are capable of making frequent, small adjustments to the speed ratio without discernable shift events. If the predetermined range of speed ratios does not include an infinite ratio (zero output speed combined with positive non-zero input speed), then some form of launch device is required in order to transition a vehicle from stationary to moving. If the predetermined range of speed ratios includes an infinite ratio, the transmission is called an Infinitely Variable Transmission (IVT).
CVTs, including IVTs, generally include a variator which includes a mechanism for adjusting the ratio of the speeds of two shafts to a desired value within some range. A CVT may include additional components that establish power flow paths among the two shafts, an input, and an output. The additional components may shift the ratio range of the transmission (between the input and the output) relative to the ratio range of the variator (between the two shafts). For example, the ratio range of the transmission may be wider than the ratio range of the variator.
FIG. 1 illustrates one type of variator called a ball variator. Four elements are supported for rotation about a central shaft 10, including an inner race 12, a left outer race 14, a right outer race 16, and a carrier 18. Since these components are essentially axi-symetric, they appear on both sides of the central axis 10 in a cross sectional depiction such as FIG. 1. Carrier 18 includes a left leg 18A and a right leg 18B. Several ball axles 20 extend between left leg 18A and right leg 18B at several circumferential locations. The ball axles 20 are connected to an actuator 22 by one or more rods 24, such that moving actuator 22 causes the axis of the ball axles 20 to rotate. For example, when actuator 22 is moved to the right as shown in FIG. 1, the right sides of the ball axles are closer to the central axis 10 than the left sides of the ball axles. Several balls 26 are supported for rotation about the ball axles 20. The inner race 12, left outer race 14, and right outer race 16 each contact each of the balls 26. The races are held against the balls with sufficient force to prevent substantial slipping at the contact points.
The ball variator constrains the relative speeds of the inner race 12, the left outer race 14, the right outer race 16, and the carrier 18. The radii, R1 through R3, of the contact points with respect to the ball axle axis is a function of the tilt angle of the ball axle. The speed difference between carrier 18 and inner race 12 is proportional to the rotational speed of ball 26 about the ball axle and proportional to R1. Similarly, the speed differences between carrier 18 and left outer race 14 and right outer race 16, respectively, are proportional to the speed of ball 26 and proportional to R2 and R3, respectively. The speed of carrier 18 is a weighted average of the speeds of inner race 12 and left outer race 14 with the weighting factors determined by the position of actuator 22. The speed of carrier 18 is also a weighted average of the speeds on inner race 12 and right outer race 16 although the weighting factors are different for all but one position of actuator 22.